We introduce a new characterization of the NP complexity class, called Quadratic Span Programs (QSPs), which is a natural extension of span programs defined by Karchmer and Wigderson. Our main motivation is the construction of succinct arguments of NP-statements that are quick to construct and verify. QSPs seem well-suited for this task, perhaps even better than Probabilistically Checkable Proofs (PCPs).

In 2010, Groth constructed a NIZK argument in the common reference string (CRS) model for Circuit-SAT consisting of only 42 elements in a bilinear group. Interestingly, his argument does not (explicitly) use PCPs. But his scheme has some disadvantages -- namely, the CRS size and prover computation are both quadratic in the circuit size. In 2011, Lipmaa reduced the CRS size to quasi-linear, but with prover computation still quadratic.

Using QSPs we construct a NIZK argument in the CRS model for Circuit-SAT consisting of just 7 group elements. The CRS size is linear in the circuit size, and prover computation is quasi-linear, making our scheme seemingly quite practical. (The prover only needs to do a linear number of group operations; the quasi-linear computation is a multipoint evaluation and interpolation.)

Our results are complementary to those of Valiant (TCC 2008) and Bitansky et al. (2012), who use \"bootstrapping\" (recursive composition) of arguments to reduce CRS size and prover and verifier computation. QSPs also provide a crisp mathematical abstraction of some of the techniques underlying Groth\'s and Lipmaa\'s constructions.

We consider designing broadcast encryption schemes with constant-size secret keys and ciphertexts, achieving chosen-ciphertext security. We first argue that known CPA-to-CCA transforms currently do not yield such schemes. We then propose a scheme, modifying a previous selective CPA secure proposal by Boneh, Gentry, and Waters. Our proposed scheme has constant-size secret keys and ciphertexts and we prove that it is selective chosen-ciphertext secure based on standard assumptions. Our scheme has ciphertexts that are shorter than those of the previous CCA secure proposals. Then we propose a second scheme that provides the functionality of both broadcast encryption and revocation schemes simultaneously using the same set of parameters. Finally we show that it is possible to prove our first scheme adaptive chosen-ciphertext secure under reasonable extensions of the bilinear Diffie-Hellman exponent and the knowledge of exponent assumptions. We prove both of these extended assumptions in the generic group model. Hence, our scheme becomes the first to achieve constant-size secret keys and ciphertexts (both asymptotically optimal) and adaptive chosen-ciphertext security at the same time.

We consider applications scenarios where an untrusted aggregator wishes to continually monitor the heavy-hitters across a set of distributed streams. Since each stream can contain sensitive data, such as the purchase history of customers, we wish to guarantee the privacy of each stream, while allowing the untrusted aggregator to accurately detect the heavy hitters and their approximate frequencies.

Our protocols are scalable in settings where the volume of streaming data is large, since we guarantee low memory usage and processing overhead by each data source, and low communication overhead between the data sources and the aggregator.

We propose a fully private fingerprint matching protocol that compares two fingerprints based on the most widely-used minutia-based fingerprint matching algorithm. The protocol enables two parties, each holding a private fingerprint, to find out if their fingerprints belong to the same individual. Unlike previous works, we do not make any simplifying assumption on the matching algorithm or use generic multiparty computation protocols in our constructions. We employ a commonly-used algorithm that works by first comparing minutia pairs from the two fingerprints based on their types, locations, and orientations, and then checking if the number of matching minutia pairs is more than a threshold, and we propose a concrete, scalable, and modular protocol. We prove security against honest-but-curious adversaries and discuss how security against malicious adversaries can be achieved using standard cryptographic techniques. Our protocol is realized using common cryptographic primitives and do not require pairing- or lattice-based cryptography.

Public-key encryption schemes rely for their IND-CPA security on per-message fresh randomness. In practice, randomness may be of poor quality for a variety of reasons, leading to failure of the schemes. Expecting the systems to improve is unrealistic. What we show in this paper is that we can, instead, improve the cryptography to offset the lack of possible randomness. We provide public-key encryption schemes that achieve IND-CPA security when the randomness they use is of high quality, but, when the latter is not the case, rather than breaking completely, they achieve a weaker but still useful notion of security that we call IND-CDA. This hedged public-key encryption provides the best possible security guarantees in the face of bad randomness. We provide simple RO-based ways to make in-practice IND-CPA schemes hedge secure with minimal software changes. We also provide non-RO model schemes relying on lossy trapdoor functions (LTDFs) and techniques from deterministic encryption. They achieve adaptive security by establishing and exploiting the anonymity of LTDFs which we believe is of independent interest.

The Gr{\\o}stl hash function is one of the 5 final round candidates of the {\\shathree} competition hosted by NIST. In this paper, we study the preimage resistance of the Gr{\\o}stl hash function. We propose pseudo preimage attacks on Gr{\\o}stl hash function for both 256-bit and 512-bit versions, i.e. we need to choose the initial value in order to invert the hash function. Pseudo preimage attack on 5(out of 10)-round Gr{\\o}stl-256 has a complexity of $(2^{244.85},2^{230.13})$ (in time and memory) and pseudo preimage attack on 8(out of 14)-round Gr{\\o}stl-512 has a complexity of $(2^{507.32},2^{507.00})$. To the best of our knowledge, our attacks are the first (pseudo) preimage attacks on round-reduced Gr{\\o}stl hash function, including its compression function and output transformation. These results are obtained by a variant of meet-in-the-middle preimage attack framework by Aoki and Sasaki. We also improve the time complexities of the preimage attacks against 5-round Whirlpool and 7-round AES hashes by Sasaki in FSE~2011.

Hummingbird is a lightweight encryption and message authentication primitive published in RISC\'09 and WLC\'10. In FSE\'11, Markku-Juhani O.Saarinen presented a differential divide-and-conquer method which has complexity upper bounded by 264 operations and requires processing of few megabytes of chosen messages under two related nonces (IVs). The improved version, Hummingbird-2, was presented in RFIDSec 2011. Based on the idea of differential collision, this paper discovers some weaknesses of the round function WD16 combining with key loading algorithm and we propose a related-key chosen-IV attack which can recover the full secret key. Under 24 pairs of related keys, the 128 bit initial key can be recovered, with the computational complexity of O(232.6) and data complexity of O(232.6). The result shows that the Hummingbird-2 cipher can\'t resist related key attack.

Description: Computer and network security systems rely on the privacy and authenticity of information, which requires implementation of cryptographic functions. Software implementations of these functions are often desired because of their flexibility and cost effectiveness. In this study, we concentrate on developing high-speed and area-efficient modular multiplication and exponentiation algorithms for number-theoretic cryptosystems.\r\nThe RSA algorithm, the Diffie-Hellman key exchange scheme and Digital Signature\r\nStandard require the computation of modular exponentiation, which is broken into a series\r\nof modular multiplications. One of the most interesting advances in modular exponentiation has been the introduction of Montgomery multiplication. We are interested in two aspects of modular multiplication algorithms: development of fast and convenient methods on a given hardware platform, and hardware requirements to achieve high-performance\r\nalgorithms.\r\nArithmetic operations in the Galois field GF(2^k) have several applications in coding\r\ntheory, computer algebra, and cryptography. We are especially interested in cryptographic applications where k is large, such as elliptic curve cryptosystems.[...]